| This research is aimed at understanding the factors governing the thermal conductivity of monolithic as well as multilayer coatings consisting of Al 2O3 and ZrO2 stabilized by 8 wt-% Y2O 3 (8YSZ), deposited by plasma spray and electron-beam physical-vapor-deposition (EB-PVD) techniques. The work is divided into two major categories.; In the first, the temperature dependence of the thermal conductivity of plasma-spray-deposited monolithic coatings and multilayer coatings consisting of Al2O3 and 8YSZ was investigated. The coatings exhibited a large reduction in thermal conductivity at all temperatures when compared to the bulk monolithic Al2O3 and Y2O 3 stabilized ZrO2 (YSZ). This reduction was found to be due to porosity as well as thermal resistance brought about by interfaces in the coatings. The largest decrease in thermal conductivity of the coatings relative to monolithic fully dense materials was found to be due to splat interfaces within each layer as well as the coating-substrate interface. On the other hand, the multilayer coatings showed little variation in thermal conductivity with the number of layers, suggesting that the influence of interlayer interfaces on heat transfer is relatively small.; In the second, single-layer and multilayer coatings consisting of Al 2O3 and 8YSZ were deposited on Ni-base superalloy substrates by electron-beam physical-vapor-deposition (EB-PVD). A columnar microstructure with (200) texture and [200] direction oriented nearly perpendicularly to the plane of the coating, was observed in the single-layer 8YSZ coating. In multilayer coatings of alternating layers of Al2O3 and 8YSZ the tendency to develop columnar structure, as well as the intensity of (200) texture, decreased as the number of layers increased. In both the single layer and the multilayer coatings, the 8YSZ layer consisted almost entirely of tetragonal (T) phase with a negligible amount of monoclinic (M) phase. This is in contrast to the microstructure of the bulk (source material used for deposition) 8YSZ, which contained significant amount of M-phase in addition to T-phase. Whereas the bulk Al2O3 consisted of α-phase, only γ-phase was found in the Al2O 3 layers of coatings. The thermal conductivity data of multilayer coatings were found to be well described by a series heat-transfer model calculation, within the limits of experimental errors involved in the measurements. |
